Transistors typically include semiconductor regions used to form the source regions and drain regions. The contact resistance between metal contact plugs and the semiconductor regions is high. Accordingly, metal silicides are formed on the surfaces of the semiconductor regions such as silicon regions, germanium regions, silicon germanium regions in order to reduce the contact resistance. The contact plugs are formed to contact the silicide regions, and the contact resistance between the contact plugs and the silicide regions are low.
A typical silicidation process includes performing a pre-clean, forming a metal layer on the surfaces of the semiconductor regions, and then performing an annealing, so that the metal layer reacts with the semiconductor regions to form the silicide regions. After the reaction, the upper portions of the metal layer may be left un-reacted. An etching step is then performed to remove the un-reacted portions of the metal layer, and the silicide regions are left.
With the increasing down-sizing of integrated circuits, the silicide regions also become increasingly smaller. Accordingly, the contact resistance of the electrical contacts becomes increasingly higher.